Process and apparatus for para-xylene production

ABSTRACT

A process of producing PX comprising providing a C 8 + feedstock, the C 8 + feedstock has C 8  hydrocarbons and C 9 + hydrocarbons, to a crystallization unit under crystallization conditions to produce a PX enriched stream having a PX concentration of at least 99.5 wt % based on the weight of the PX enriched stream, wherein the C 8 + feedstock has a PX concentration of at least 70 wt % based on total weight of xylenes in the C 8 + feedstock, which the C 8 + feedstock having a C 9 + hydrocarbons concentration in a range from 1 wppm to 10 wt % based on the total weight of the C 8 + feedstock.

PRIORITY CLAIM

This application is a Continuation Application of application Ser. No.12/042,433, filed Mar. 5, 2008, now U.S. Pat. No. 7,989,672 and claimsthe benefit of U.S. Provisional Application No. 60/921,729, filed Apr.4, 2007, the entirety of which is incorporated by reference.

FIELD OF THE INVENTION

The present disclosure relates to a process for the production ofpara-xylene using a combination of a high selective toluenedisproportionation process which produces a C₈+ stream and acrystallization process that separates the para-xylene from the C₈+stream.

BACKGROUND

The C₈ alkylbenzenes, ethylbenzene (EB), para-xylene (PX), ortho-xylene(OX) and meta-xylene (MX) are often present together in a typicalindustrial C₈ aromatic product stream from a chemical plant or arefinery. For instance, commercially available PxMax, Mobil SelectiveToluene Disproportionation and Mobil Toluene Disproportionationprocesses may produce such a stream.

Of the three xylene isomers, PX has the largest commercial market. PX isused primarily for manufacturing purified terephthalic acid (PTA) andterephthalate esters such as dimethyl terephthalate (DMT), which areused for making various polymers such as poly(ethylene terephthalate),or PET, polypropylene terephthalate), or PPT, and poly(buteneterephthalate), or PBT. Different grades of PET are used for manydifferent popular consumer goods such as films, synthetic fibers, andplastic bottles for soft drinks PPT and PBT may be used for makingsimilar products with different properties.

Fractional distillation is a commonly used method for many processes inmany industrial plants to separate chemicals. However, it is oftendifficult to use such a conventional fractional distillation technologyto separate the EB and different xylene isomers efficiently andeconomically because the boiling points of the four C₈ aromatics fallwithin a very narrow 8° C. range, from about 136° C. to about 144° C.(see Table 1). The boiling points of PX and EB are about 2° C. apart.The boiling points of PX and MX are only about 1° C. apart. As a result,large equipment, significant energy consumption, and/or substantialrecycles would be required to provide effective and satisfactory xyleneseparations.

TABLE 1 C₈ compound Boiling Point (° C.) Freezing Point (° C.) EB 136−95 PX 138 13 MX 139 −48 OX 144 −25

Fractional crystallization in a crystallizer takes advantage of thedifferences between the freezing points and solubilities of the C₈aromatic components at different temperatures. Due to its higherfreezing point, PX is usually separated as a solid in such a processwhile the other components are recovered in a PX-depleted filtrate. HighPX purity, a key property needed for satisfactory commercial conversionof PX to PTA and/or DMT in most plants, can be obtained by this type offractional crystallization. U.S. Pat. No. 4,120,911 provides adescription of this method. A crystallizer that may operate in thismanner is described in U.S. Pat. No. 3,662,013. Commercially availableprocesses and crystallizers include the crystallization isofiningprocess, the continuous countercurrent crystallization process, thedirect contact CO₂ crystallizer, and the scraped drum crystallizer. Dueto high utility usage and the formation of a eutectic between PX and MX,it is usually more advantageous to use a feed with as high an initial PXconcentration as possible when using fractional crystallization torecover PX.

The term “shape-selective catalysis” describes unexpected catalyticselectivities in zeolites. The principles behind shape selectivecatalysis have been reviewed extensively, e.g., by N.Y. Chen, W. E.Garwood and F. G. Dwyer, “Shape Selective Catalysis in IndustrialApplications,” 36, Marcel Dekker, Inc. (1989). Within a zeolite pore,hydrocarbon conversion reactions such as paraffin isomerization, olefinskeletal or double bond isomerization, oligomerization and aromaticdisproportionation, alkylation or transalkylation reactions are governedby constraints imposed by the channel size. Reactant selectivity occurswhen a fraction of a feedstock is too large to enter the zeolite poresto react; while product selectivity occurs when some of the productscannot leave the zeolite channels. Product distributions can also bealtered by transition state selectivity in which certain reactionscannot occur because the reaction transition state is too large to formwithin the zeolite pores or cages. Another type of selectivity resultsfrom configurational constraints on diffusion where the dimensions ofthe molecule approach that of the zeolite pore system. A small change inthe dimensions of the molecule or the zeolite pore can result in largediffusion changes leading to different product distributions. This typeof shape selective catalysis is demonstrated, for example, in selectivetoluene disproportionation to p-xylene.

The production of PX is typically performed by toluenedisproportionation over a catalyst under conversion conditions. Examplesinclude the toluene disproportionation, as described by Pines in “TheChemistry of Catalytic Hydrocarbon Conversions”, Academic Press, N.Y.,1981, p. 72. Such methods typically result in the production of amixture including PX, OX, and MX. Depending upon the degree ofselectivity of the catalyst for PX (para-selectivity) and the reactionconditions, different percentages of PX are obtained. The yield, i.e.,the amount of xylene produced as a proportion of the feedstock, is alsoaffected by the catalyst and the reaction conditions.

The equilibrium reaction for the conversion of toluene to xylene andbenzene products normally yields about 24% PX, about 54% MX, and about22% OX among xylenes.

Conventionally, PX production by toluene disproportionation comprises:

-   -   a) toluene disproportionation step to produce a product stream        having C₇− hydrocarbons including benzene and toluene, C₈        hydrocarbons including PX, MX, OX, and ethylbenzene, and C₉+        hydrocarbons;    -   b) a separation system comprising:        -   1. a C₇− separation step to separate the C₇− hydrocarbons            from the product stream to form a C₇− depleted stream; and a            C₉+ separation step to separate the C₉+ hydrocarbons from            the C₇− depleted stream to form a C₇− and C₉+ depleted            stream which is enriched with C₈ hydrocarbons as comparing            with the product stream; or        -   2. a C₉+ separation step to separate the C₉+ hydrocarbons            from the product stream to form a C₉+ depleted stream; and a            C₇− separation step to separate the C₇− hydrocarbons from            the C₉+ depleted stream to form a C7− and C₉+ depleted            stream which is enriched with C₈ hydrocarbons as comparing            with the product stream; or        -   3. a C₇− and C₉+ separation step to separate C₇− and C₉+            hydrocarbons from the product stream to form a C₇− and C₉+            depleted stream which is enriched with C8 hydrocarbons as            comparing with the product stream; and    -   c) a PX separation step to separate PX from at least a portion        of the C₇− and C₉+ depleted stream.

Conveniently, the PX separation step (c) normally comprises acrystallization step to produce a PX product with desired purity, e.g.,at least 99 wt %. At least a portion of the C₇− and C₉+ depleted streamis used as a feedstock for the PX separation step (c). Depending on thedesired purity of the PX product and depending on the PX concentrationin the C₇− and C₉+ depleted stream, a multi-stage crystallization unitor a multi-stage adsorption unit may be needed.

Crystallization methods can be used to separate PX (p-xylene) from a C8aromatic starting material which contains ethylbenzene, as well as thethree xylene isomers. PX has a freezing point of 13.3° C., MX has afreezing point of −47.9° C. and OX has a freezing point of −25.2° C.However, conventional crystallization methods can be used to make PXwith a purity of over 99.5 wt. % only with great expense.

Crystallization processes to recover PX from a mixture of C₈ aromaticsrequires cooling the feed mixture. Because its melting point is muchhigher than that of the other C₈ aromatics, PX is readily separated inthe crystallizer after refrigeration of the stream. In conventional PXcrystallization processes, the feed contains about 22 to about 23 wt. %PX. This is the type of feed that is generally obtained from catalyticreforming of naphtha, xylene isomerization, and non-shape selectivetoluene disproportionation (TDP) processes, in which the relativeproportion of xylene isomers is close to equilibrium at reactiontemperatures. For the production of high purity PX (>99.5 to >99.8 wt %)from these feeds, these feeds are cooled, crystallized and separated ata very cold temperature, normally −65 to −70.5° C. In order to recovermost of the PX from solution, the feeds sometimes have to be cooled toas low as about −85° to −95° F. The crystals are melted, and theresulting solution is recrystallized and separated at a warmertemperature for maximum PX purity. Because of the constraint imposed bythe eutectic temperature, PX recovery from conventional crystallizationprocesses is generally limited to about 60-65%. Therefore, theseprocesses generally have less favorable economics compared to the neweradsorption based PX recovery technologies, which can recover 97-98% ofthe feed PX, and have lower capital and operating costs.

U.S. Pat. No. 5,448,005 discloses a crystallization process for PXrecovery. A single temperature crystallization production stage is usedfor producing PX from a feed having a PX concentration aboveequilibrium, such as from a toluene disproportionation process.Scavenger stages are also used to raise the PX recovery rate.

U.S. Pat. No. 5,498,822 discloses a crystallization process for PXrecovery. A single temperature crystallization stage is used forproducing PX from a feed having an above equilibrium PX concentration,such as from toluene disproportionation.

Various methods are known in the art for increasing the para-selectivityof zeolite catalysts, for example, U.S. Pat. Nos. 5,349,113, 5,498,814,5,349,114, 5,476,823, 5,367,099, 5,403,800, 5,365,004, 5,610,112,5,455,213, 5,516,736, 5,495,059, 5,633,417, 5,659,098, 6,576,582 and6,777,583.

A modified crystallization process (WO95/26946) may be used when thefeed contains a relatively high concentration of PX. The C₈ aromaticmixture obtained from selective toluene disproportionation (STDP)processes generally contains over 70 wt % PX. For this type of feed,high recovery of PX is possible using a single production stage atrelatively high temperature, −17.8° C. to 10° C. The filtrate isprocessed through one or more scavenger stages operating at lowertemperature, −28.9° C. to −1.1° C., to recover additional PX, which isrecycled to the production stage for final purification. When the C₈aromatic mixture contains over 97% PX, it is possible to obtain over 90%recovery in a single production stage operating at −28.9° C. to 10° C.,with no scavenger stage (WO95/26947). Such mixtures may be obtained fromSTDP processes using a silica modified catalyst.

Because of their reduced refrigeration requirements and greaterpotential recovery of PX, these modified crystallization processes aregenerally competitive with adsorption based processes. It is believedthat the feedstock to the crystallization step (c) requires very lowlevel of C₉+ hydrocarbons, which may interfere with the performance ofthe crystallization unit. Therefore, a C₉+ separation step is requiredto remove C₉+ from the product stream of step (a), normally a C₉+distillation column is needed to achieve desired C₉+ level in afeedstock for the PX separation step (c).

It has now been surprisingly found that the C₉+ separation step may beeliminated or minimized by the combination of high selective toluenedisproportionation process which produces a C₈ stream and acrystallization process. The elimination or minimization of the C₉+separation step can reduce energy consumption, capital cost, operationalcost, and emission to the environment for a PX production plant, whichwill translate to low PX cost of production and less emission to thelocal environment.

SUMMARY OF THE DISCLOSURE

In some embodiments, this disclosure relates to a process of producingPX comprising providing a C₈+ feedstock, having C₈ hydrocarbons and C₉+hydrocarbons, to a crystallization unit under crystallization conditionsto produce a PX enriched stream having a PX concentration of at least99.5 wt % based on the weight of the PX enriched stream, wherein the C₈+feedstock has a PX concentration of at least 70 wt % based on totalweight of xylenes in the C₈+ feedstock, which the C₈+ feedstock having aC₉+ hydrocarbons concentration in a range from 1 wppm to 10 wt % basedon the total weight of the C₈+ feedstock.

In other embodiments, this disclosure relates to a process of producingPX comprising: (a) providing a toluene feedstock having toluene to areaction zone; (b) contacting the toluene with a catalyst under toluenedisproportionation conditions to form an effluent having C₇−hydrocarbons, C₈ hydrocarbons and C₉+ hydrocarbons, wherein the C₈hydrocarbons comprise PX, MX, and OX, wherein the effluent has a PXconcentration of at least 70 wt % based on total weight of xylenes inthe effluent; (c) separating at least a portion of C₇− hydrocarbons fromthe effluent to from a C₈+ feedstock, wherein the C₈+ feedstock has aC₉+ hydrocarbons concentration from 1 wppm to 10 wt % based on the totalweight of the C₈+ feedstock; and (d) supply at least a portion of theC₈+ feedstock to a crystallization unit under crystallization conditionsto produce a PX enriched stream having a PX concentration of at least99.5 wt % based on the weight of the PX enriched stream.

In some aspects of this disclosure, the feedstock supplied to thecrystallization unit is made by a STDP process consisting essentiallyof:

-   -   (a) a toluene purifying step to produce a toluene feedstock        comprising at least 90 wt. % toluene and non-aromatic        hydrocarbons ranging from 1 to 10 wt % based on the weight of        the toluene feedstock; wherein the toluene purifying step has        feed(s) comprises an aromatic product stream from a catalytic        reformer, an aromatic product stream from a catalytic cracker,        and/or an aromatic product stream from a steam cracker, wherein        the aromatic product stream from a catalytic reformer, the        aromatic product stream from a catalytic cracker, or the        aromatic product stream from a steam cracker comprises at least        1 wppm to about 15 wt % non-aromatic hydrocarbons;    -   (b) contacting the toluene feedstock with a catalyst under        toluene disproportionation conditions to product a toluene        disproportionation product having light gases, Bz, PX, MX, OX,        C₉+ and unreacted toluene, wherein the toluene        disproportionating step has a toluene conversion ranging from        about 15 to 35 wt % based on the toluene in the toluene        feedstock, and wherein the toluene disproportionation product        has a PX concentration of at least 70 wt % based on total        xylenes in the toluene disproportionation product; and    -   (c) separating at least a portion of the light gases, at least a        portion of the Bz, and at least a portion of the unreacted        toluene from the toluene disproportionation product to produce        the feedstock of any one of claims 1, 2, 3, 5, 6, and 7.

In some embodiments, this disclosure relates to a process of producingPX consisting essentially of:

-   -   (a) a toluene purifying step to produce a toluene feedstock        comprising at least 90 wt. % toluene and non-aromatic        hydrocarbons ranging from 1 to 10 wt % based on the weight of        the toluene feedstock; wherein the toluene purifying step has        feed(s) comprises an aromatic product stream from a catalytic        reformer, an aromatic product stream from a catalytic cracker,        and/or an aromatic product stream from a steam cracker, wherein        the aromatic product stream from a catalytic reformer, the        aromatic product stream from a catalytic cracker, or the        aromatic product stream from a steam cracker comprises at least        1 wppm to about 15 wt % non-aromatic hydrocarbons;    -   (b) contacting the toluene feedstock with a catalyst under        toluene disproportionation conditions to product a toluene        disproportionation product having light gases, Bz, PX, MX, OX,        C₉+ and unreacted toluene, wherein the toluene        disproportionating step has a toluene conversion ranging from        about 15 to 35 wt % based on the toluene in the toluene        feedstock, and wherein the toluene disproportionation product        has a PX concentration of at least 70 wt % based on total        xylenes in the toluene disproportionation product;    -   (c) separating at least a portion of the light gases, at least a        portion of the Bz, and at least a portion of the unreacted        toluene from the toluene disproportionation product to produce a        C₈+ feedstock; and    -   (d) providing the C₈+ feedstock to a crystallization unit under        crystallization conditions to produce a PX enriched stream        having a PX concentration of at least 99.7 wt % based on the        weight of the PX enriched stream, wherein the C₈+ feedstock has        a PX concentration of at least 70 wt % based on total weight of        xylenes in the C₈+ feedstock, which the C₈+ feedstock having a        C₉+ hydrocarbons concentration in a range from 5000 wppm to 10        wt % based on the total weight of the C₈+ feedstock.

In additional embodiments, this disclosure relates to an apparatus forproducing a PX rich stream, which comprises: (a) a reactor having aninlet and an outlet; (b) a separation unit having an inlet and a firstoutlet and a second outlet, the inlet of the separation unit beingfluidicly connected to the outlet of the reactor; and (c) acrystallization unit having an inlet, a first outlet, and a secondoutlet, the inlet of the crystallization unit being fluidicly connectedto the second outlet of the separation unit.

DETAILED DESCRIPTION

As used in this specification, the term “framework type” is used in thesense described in the “Atlas of Zeolite Framework Types,” 2001.

As used herein, the numbering scheme for the Periodic Table Groups isused as in Chemical and Engineering News, 63 (5), 27 (1985).

The term “wppm” as used herein is defined as parts per million byweight.

All weights of molecular sieve, weights of binder, and weights ofcatalyst composition, as used in herein, are based on the calcinedweight (i.e., calcined at 510° C. in air for at least one hour).

The term “C_(n)” hydrocarbon wherein n is an positive integer, e.g., 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein means a hydrocarbonhaving n number of carbon atom(s) per molecular. For example, C_(n)aromatics means an aromatic hydrocarbon having n number of carbonatom(s) per molecular; C_(n) paraffin means a paraffin hydrocarbonhaving n number of carbon atom(s) per molecular; C_(n) olefin means anolefin hydrocarbon having n number of carbon atom(s) per molecular. Theterm “C_(n)+” hydrocarbon wherein n is an positive integer, e.g., 1, 2,3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein means a hydrocarbonhaving at least n number of carbon atom(s) per molecular. The term“C_(n)−” hydrocarbon wherein n is an positive integer, e.g., 1, 2, 3, 4,5, 6, 7, 8, 9, 10, 11, 12, as used herein means a hydrocarbon having nomore than n number of carbon atom(s) per molecular.

The term “C_(n)+” feedstock, wherein n is an positive integer, e.g., 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein means a feedstockcomprising a majority (greater than 50 wt % based on the total weight ofthe feedstock) hydrocarbons having at least n number of carbon atom(s)per molecular. The term “C_(n)−” feedstock wherein n is an positiveinteger, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used hereinmeans a feedstock comprising a majority (greater than 50 wt % based onthe total weight of the feedstock) hydrocarbons having no more than nnumber of carbon atom(s) per molecular.

In some embodiments, this disclosure relates to a process of producingPX comprising providing a C₈+ feedstock, having C₈ hydrocarbons and C₉+hydrocarbons, to a crystallization unit under crystallization conditionsto produce a PX enriched stream having a PX concentration of at least99.5 wt % based on the weight of the PX enriched stream, wherein the C₈+feedstock has a PX concentration of at least 70 wt % based on totalweight of xylenes in the C₈+ feedstock, which the C₈+ feedstock having aC₉+ hydrocarbons concentration in a range from 1 wppm to 10 wt % basedon the total weight of the C₈+ feedstock.

The term “non-aromatic” hydrocarbon means a hydrocarbon having noaromatic ring. Examples of non-aromatic hydrocarbon are paraffin(s),olefin(s), cyclic paraffin(s), or cyclic olefin(s).

In other embodiments, this disclosure relates to a process of producingPX comprising: (a) providing a toluene feedstock having toluene to areaction zone; (b) contacting the toluene with a catalyst under toluenedisproportionation conditions to form an effluent having C₇−hydrocarbons, C₈ hydrocarbons and C₉+ hydrocarbons, wherein the C₈hydrocarbons comprise PX, MX, and OX, wherein the effluent has a PXconcentration of at least 70 wt % based on total weight of xylenes inthe effluent; (c) separating at least a portion of C₇− hydrocarbons fromthe effluent to from a C₈+ feedstock, wherein the C₈+ feedstock has aC₉+ hydrocarbons concentration from 1 wppm to 10 wt % based on the totalweight of the C₈+ feedstock; and (d) supply at least a portion of theC₈+ feedstock to a crystallization unit under crystallization conditionsto produce a PX enriched stream having a PX concentration of at least99.5 wt % based on the weight of the PX enriched stream.

C₈+ Feedstock

The C₈+ feedstock useful for this disclosure has C₈ hydrocarbons and C₉+hydrocarbons. In some embodiment, a C₈+ feedstock useful for thisdisclosure is produced by separation/purification from a hydrocarbonstream made in a reforming process, a hydrocracking process, a toluenedisproportionation process, a selective toluene disproportionationprocess, a toluene methylation process, or any combination thereof. TheC₈+ feedstock useful for this disclosure has a PX concentration of atleast 70 wt % based on total weight of xylenes in the C₈+ feedstock andthe C₈+ feedstock has a C₉+ hydrocarbons concentration in a range from 1wppm to 10 wt % based on the total weight of the C₈+ feedstock

The following PX concentration, in wt % based on total weight of xylenesin a C₈+ feedstock, are useful lower PX concentration limits for alldisclosure processes: 70, 75, 80, 85, 89, 93, and 95. The following PXconcentration, in wt % based on total weight of xylenes in the C₈+feedstock, are useful upper PX concentration limits for all disclosureprocesses: 99, 98, 97, 96, 95, and 90. The PX concentration, in wt %based on total weight of xylenes in the C₈+ feedstock may be present inan amount ranging from 70 wt % to 99 wt % in one embodiment,alternatively 75 wt % to 98 wt %, alternatively from 80 wt % to 97 wt %,alternatively 85 to 95 wt %, alternatively 85 wt % to 99 wt %,alternatively and from 85 wt % to 95 wt % in another embodiment.

The following C₉+ hydrocarbons concentration, based on the total weightof the C₈+ feedstock, are useful lower C₉+ hydrocarbons concentrationlimits for all disclosure processes: 1 wppm, 2 wppm, 5 wppm, 10 wppm, 50wppm, 100 wppm, 200 wppm, 500 wppm, 1000 wppm, 2000 wppm, 5000 wppm, 1wt %, 2 wt % and 5 wt %. The following C₉+ hydrocarbons concentration,based on the total weight of the C₈+ feedstock, are useful upper C₉+hydrocarbons concentration limits for all disclosure processes: 100wppm, 200 wppm, 500 wppm, 1000 wppm, 2000 wppm, 5000 wppm, 1 wt %, 2 wt%, 5 wt % and 10 wt %. The C₉+ hydrocarbons concentration, based on thetotal weight of the C₈+ feedstock may be present in an amount rangingfrom 1 wppm to 10 wt % in one embodiment, alternatively 10 wppm to 5 wt%, alternatively from 20 wppm to 2 wt %, alternatively 1 wppm to 1 wt %,alternatively 2 wppm to 1 wt %, alternatively and from 5 wppm to 1 wt %in another embodiment.

In some embodiments, the C₈+ feedstock may further comprise naphthalene.When naphthalene is present in the C₈+ feedstock, the naphthaleneconcentration, in mol %, based on the PX free C₈+ in the C₈+ feedstockis ranging from about 0.0001 to 10 mol %. The following naphthaleneconcentration, in mol %, based on the PX free C₈+ hydrocarbons in a C₈+feedstock, are useful lower naphthalene concentration limits for alldisclosure processes: 0.0001, 0.0002, 0.0005, 0.001, 0.005, 0.01, 0.02,0.05, 0.1, 0.2, 0.5, 1 mol %, 2 mol % and 5 mol %. The followingnaphthalene concentration, based on the PX free C₈+ in the C₈+feedstock, are useful upper naphthalene concentration limits for alldisclosure processes: 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1 mol %, 2 mol %,5 mol % and 10 mol %. The naphthalene concentration based on the PX freeC₈+ hydrocarbons in a C₈+ feedstock may be present in an amount rangingfrom 0.0001 to 10 mol % in one embodiment, alternatively 0.001 to 5 mol%, alternatively from 0.002 to 2 mol %, alternatively 0.0001 to 1 mol %,alternatively 0.0002 to 1 mol %, alternatively and from 0.0005 to 1 mol% in another embodiment. The naphthalene concentration, in mol %, basedon the PX free C₈+ hydrocarbons in a C₈+ feedstock is calculated withthe following equation:

${{naphthalene}\mspace{14mu}{concentration}} = \frac{\begin{matrix}{{total}\mspace{14mu}{mole}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{naphthalene}\mspace{14mu}{in}} \\{{{the}\mspace{14mu} C_{8}} + {{feedstock} \times 100}}\end{matrix}}{\begin{matrix}{{{Mole}\mspace{14mu}{of}\mspace{14mu} C_{8}} + {{hydrocarbons}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu} C_{8}} +} \\{{feedstock} - {{Mole}\mspace{14mu}{of}\mspace{14mu}{PX}\mspace{14mu}{in}\mspace{14mu}{the}\mspace{14mu} C_{8}} + {feedstock}}\end{matrix}}$PX Enriched Stream Product

In one embodiment, the PX enriched stream produced by thecrystallization unit has a PX concentration of at least 99.5 wt % basedon the weight of the PX enriched stream. The following PX concentrationof the PX enriched stream, in wt %, based on the weight of the PXenriched stream, are useful lower PX concentration limits for alldisclosure processes: 95, 96, 97, 98, 99, 99.5, 99.6, 99.7, 99.8, 99.85,99.86, 99.87, 99.88 and 99.9. The following PX concentration of the PXenriched stream, in wt %, based on the weight of the PX enriched stream,are useful upper PX concentration limits for all disclosure processes:99.8, 99.85, 99.86, 99.87, 99.88, 99.89, 99.9, 99.95 and 99.999999. Thefollowing PX concentration of the PX enriched stream, in wt %, based onthe weight of the PX enriched stream, may be present in an amountranging from 95 to 100 in one embodiment, alternatively 98 to 9.99,alternatively from 99 to 99.99, alternatively 99.5 to 99.99,alternatively 99.6 to 99.99, alternatively and from 99.7 to 99.99 inanother embodiment.

Crystallization Conditions

In some embodiments, the crystallization unit is normally operated at atemperature of at least 1° C. higher than the highest eutectic point ofPX with MX, PX with naphthalene (when naphthalene is present in the C₈+feedstock) or PX with other C₈+ hydrocarbons. In other embodiments, thecrystallization unit is operated at a temperature of at least 2° C., 3°C., 4° C., 5° C., 6° C., 7° C., 8° C., 9° C., 10° C., 15° C., or 20° C.higher than the highest eutectic point of PX with MX, PX withnaphthalene (when naphthalene is present in the C₈+ feedstock) or PXwith other C₈+ hydrocarbons. In other embodiments, the crystallizationunit is operated at a temperature of no more than 100° C., 50° C., 40°C., 30° C., or 20° C. higher than the highest eutectic point of PX withMX, PX with naphthalene (when naphthalene is present in the C₈+feedstock) or PX with other C₈+ hydrocarbons. In some embodiments, thecrystallization unit is operated at a temperature ranging from 1° C. to100° C., alternatively, 2° C. to 50° C., alternatively, 5° C. to 50° C.,alternatively, 5° C. to 30° C., or alternatively, 5° C. to 20° C.

In a preferred embodiment, the crystallization unit is operated at atemperature of at least −30° C., alternatively −25° C., alternatively−20° C., alternatively −18° C., alternatively −15° C., alternatively−10° C., alternatively −5° C., alternatively 0° C., alternatively 5° C.,or alternatively 10° C.

In some embodiments, when the C₈+ feedstock having a PX concentration ofgreater than 70 wt % based on the total xylenes in the C₈+ feedstock,the crystallization unit is operated using a single production stage atrelatively high temperature, −17.8° C. to 10° C., wherein the filtrateis processed through one or more scavenger stages operating at lowertemperature, −28.9° C. to −1.1° C., to recover additional PX, which isrecycled to the production stage for final purification as disclosed inWO95/26946. Alternatively, when the C₈+ feedstock having a PXconcentration of greater than 97 wt % based on the total xylenes in theC₈+ feedstock, the crystallization unit may be operated using a singleproduction stage operating at −28.9° C. to 10° C., with no scavengerstage as disclosed in WO95/26947. The entirety of WO95/26946 andWO95/26947 are incorporated by reference.

The temperatures referenced above pertain to the coldest crystallizerstage. In practice, it is known that two or three stages at a range oftemperatures are needed to obtain an acceptably high purity PX product.However, the temperature of the coldest stage is significant because itdetermines the maximum PX recovery that may be obtained.

PX depleted filtrate is recycled to each stage to control the solidscontent of the crystallizer effluent that is sent to the liquid-solidseparation apparatus, normally a centrifuge. The final crystallineproduct is washed with a liquid, preferably high purity PX product, todisplace the residual filtrate from the wet cake.

In some embodiments, the crystallization unit is operated with a PXrecovery of at least 65 wt %. The following PX recovery, in wt %, basedon the PX in C₈+ feedstock, are useful lower PX recovery limits for alldisclosure processes: 70, 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98and 99. The following PX recovery, in wt %, based on the weight of thePX in the C₈+ feedstock, are useful upper PX recovery limits for alldisclosure processes: 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99and 99.99. The following PX recovery, in wt %, based on the weight ofthe PX in the C₈+ feedstock, may be present in an amount ranging from 95to 100 in one embodiment, alternatively 70 to 99.9, alternatively from80 to 99, alternatively 85 to 99.99, alternatively 90 to 99.99,alternatively and from 95 to 99.99 in another embodiment.

Selective Toluene Disproportionation

STDP provides a process for obtaining p-xylene at toluene conversions ofat least 10 wt %, preferably at least about 15-25 wt %, with a PXselectivity of greater than 85 wt %, preferably at least 90 wt % basedon the total xylenes in the product.

The toluene feedstock may be produced by any separation technique, suchas, distillation of a feed containing toluene. Examples of a feedcontaining toluene are an aromatic product stream of a catalyticreformer, an aromatic product stream of a catalytic cracker, an aromaticproduct stream of a steam cracker, or any combination thereof. In someaspects, the aromatic product stream of a catalytic reformer, thearomatic product stream of a catalytic cracker, or the aromatic productstream of a steam cracker may optionally subject to extraction to removenon-aromatic hydrocarbons from said aromatic product stream(s). In otheraspects, the aromatic product stream of a catalytic reformer, thearomatic product stream of a catalytic cracker, or the aromatic productstream of a steam cracker may not subject to extraction process withoutremoving non-aromatic hydrocarbons from said aromatic product stream(s).The non-aromatic hydrocarbons content in the toluene feedstock is afunction of the feed composition and separation technique/efficiencyused to produce the toluene feedstock.

The toluene feedstock preferably includes about 50 wt % to 100 wt %toluene, more preferably at least about 80 wt % toluene based on thetotal weight of the toluene feedstock. Other compounds such as benzene,xylenes, trimethylbenzene, and non-aromatics may also be present in thetoluene feedstock without adversely affecting the present disclosure.The amount of the non-aromatics may be in a range from about 1 wppm to15 wt % based on the total weight of the toluene feedstock. Thefollowing non-aromatics, in wt %, based on the total weight of thetoluene feedstock, are useful lower non-aromatics limits for alldisclosure processes: 0.01, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11, 12, 13 and 14. The following non-aromatics, in wt %, based onthe total weight of the toluene feedstock, are useful highernon-aromatics limits for all disclosure processes: 1, 2, 3, 4, 5, 6, 7,8, 9, 10, 11, 12, 13, 14 and 15. The following non-aromatics, in wt %,based on the total weight of the toluene feedstock, may be present in anamount ranging from 0.1 to 15 in one embodiment, alternatively 1 to 10,alternatively from 3 to 15, alternatively 3 to 10, alternatively 4 to15, alternatively and from 4 to 10 in another embodiment.

The toluene feedstock may also be dried, if desired, in a manner whichwill minimize moisture entering the reaction zone. Numerous methodsknown in the art are suitable for drying the toluene charge for theprocess of the disclosure. These methods include percolation through anysuitable desiccant, for example, silica gel, activated alumina,molecular sieves or other suitable substances, stripping, distillation,and/or the use of liquid charge dryers.

The catalytic molecular sieves useful in accordance with the methods ofthe present disclosure are preferably in the hydrogen form prior tomodification, but may be in the ammonium or sodium form. Preferably, thecatalytic molecular sieve comprises an intermediate pore-size molecularsieve such as a ZSM-5, ZSM-11, ZSM-22, ZSM-23, or ZSM-35 as discussedabove. The catalytic molecular sieves also preferably have a ConstraintIndex of about 1-12. The details of the method by which Constraint Indexis determined are described fully in U.S. Pat. No. 4,016,218,incorporated herein by reference.

The crystal size of molecular sieves used herein is preferably greaterthan 0.1 micron. The accurate measurement of crystal size of molecularsieve materials is frequently very difficult. Microscopy methods, suchSEM and TEM, are often used, but these methods require measurements on alarge number of crystals and for each crystal measured, values may berequired in up to three dimensions. For ZSM-5 materials described in theexamples below, estimates were made of the effective average crystalsize by measuring the rate of sorption of 2,2-dimethylbutane at 90° C.and 8 kPa-a hydrocarbon pressure. The crystal size is computed byapplying the diffusion equation given by J. Crank, “The Mathematics ofDiffusion” Oxford at the Clarendon Press, 1957, pp 52-56, for the rateof sorbate uptake by a solid whose diffusion properties can beapproximated by a plane sheet model. In addition, the diffusion constantof 2,2-dimethylbutane, D, under these conditions is taken to be1.5×10⁻¹⁴ cm²/sec.

The catalyst for STDP may be a catalyst selectivated by coke, silicon,metal(s), or any combination thereof.

Operating conditions employed in the process of the present disclosurewill affect the para-selectivity and toluene conversion. Such conditionsinclude the temperature, pressure, space velocity, molar ratio of thereactants, and the hydrogen to hydrocarbon mole ratio (H₂/HC). It hasalso been observed that an increased space velocity (WHSV) can enhancethe para-selectivity of the modified catalyst in alkylbenzenedisproportionation reactions. This characteristic of the modifiedcatalyst allows for substantially improved throughput when compared tocurrent commercial practices. In addition, it has been observed that thedisproportionation process may be performed using H₂ as a diluent,thereby dramatically increasing the cycle length of the catalyst. Forexample, it has been observed that an increase in temperature canincrease the activity of the modified catalyst.

A selectivated and steamed catalytic molecular sieve may be contactedwith a toluene feedstock under conditions for effecting vapor-phasedisproportionation. Conditions effective for accomplishing the highpara-selectivity and acceptable toluene disproportionation conversionrates include a reactor inlet temperature of from about 200° C. to about600° C., preferably from 350° C. to about 540° C.; a pressure of fromabout 101.3 kPa-a to about 34.48 MPa-a, preferably from about 689 kPa-ato about 6.89 MPa-a; a WHSV of from about 0.1 to about 20 hr⁻¹,preferably from about 2 to about 10 hr⁻¹; and a H₂/HC mole ratio of fromabout 0.1 to about 20, preferably from about 2 to about 6. This processmay be conducted in either batch or fluid bed operation, with theattendant benefits of either operation readily obtainable. The effluentmay be separated and distilled to remove the desired product, i.e.,p-xylene, as well as other by-products. Alternatively, the C₈ fractionmay be subjected to further separation, as in the case of xylenes,subjected to crystallization process to yield p-xylene.

The catalyst may be further modified in order to reduce the amount ofundesirable by-products, particularly ethylbenzene. The state of the artis such that the reactor effluent from standard toluenedisproportionation typically contains about 0.5% ethylbenzeneby-product. Upon distillation of the reaction products, the level ofethylbenzene in the C₈ fraction often increases to between about 3% and4%. This level of ethylbenzene is unacceptable for polymer gradep-xylene, since ethylbenzene in the p-xylene product, if not removed,degrades the quality of fibers ultimately produced from the p-xyleneproduct. Consequently, ethylbenzene content of the p-xylene product mustbe kept low. The specification for the allowable amount of ethylbenzenein the p-xylene product has been determined by the industry to be lessthan 0.3%. Ethylbenzene can be substantially removed by crystallizationor by superfractionation processes.

In order to avoid the need for downstream ethylbenzene removal, thelevel of ethylbenzene by-product is advantageously reduced byincorporating a hydrogenation/dehydrogenation function within thecatalyst, such as by addition of a metal compound or metal compound(s),such as platinum or platinum/tin. While platinum is the preferred metal,other metals of Groups 6 to 12 of the Periodic Table such as palladium,nickel, copper, cobalt, molybdenum, rhodium, ruthenium, silver, gold,mercury, osmium, iron, zinc, cadmium, and mixtures thereof, may beutilized. The metal may be added by cation exchange, in amounts of fromabout 0.001 wt % to about 2 wt %, typically about 0.5 wt %. For example,a platinum modified catalyst can be prepared by first adding thecatalyst to a solution of ammonium nitrate in order to convert thecatalyst to the ammonium form. The catalyst is subsequently contactedwith an aqueous solution of tetraamine platinum(II) nitrate ortetraamine platinum(II) chloride. The catalyst can then be filtered,washed with water and calcined at temperatures of from about 250° C. toabout 500° C. It will be appreciated by those skilled in the art thatsimilar considerations apply to processes involving alkylbenzenes otherthan toluene.

Apparatus for PX Production

In additional embodiments, this disclosure relates to an apparatus forproducing a PX rich stream, which comprises: (a) a reactor having aninlet and an outlet; (b) a separation unit having an inlet and a firstoutlet and a second outlet, the inlet of the separation unit beingfluidicly connected to the outlet of the reactor; and (c) acrystallization unit having an inlet, a first outlet, and a secondoutlet, the inlet of the crystallization unit being fluidicly connectedto the second outlet of the separation unit.

A typical toluene disproportionation process comprises a toluenedisproportionation reactor. Toluene feedstock is fed to the toluenedisproportionation reactor optionally co-fed with H₂. The productnormally comprises H₂, C₇− hydrocarbons (including benzene), C₈hydrocarbons (including PX, MX, and OX), and C₉+ hydrocarbons. Theproduct stream is conveniently supplied to a series of fractionationunits to separate H₂ which may be recycled to the toluenedisproportionation reactor, light gases, benzene as one final product,and toluene which may recycle back to the toluene disproportionationreactor. The rest of the product after separation of H₂, light gases,benzene and toluene, namely C₈+ product stream, contains mainly xylenesand C₉+ hydrocarbons. Conveniently, the C₉+ is separated from thexylenes before further separation of PX at a crystallization unit.

In some embodiments of this disclosure, the C₈+ product stream is fed toa crystallizer without the C₉+ separation step. By eliminating the C₉+separation step, e.g., eliminating a distillation tower for separatingxylenes from C₉+, huge energy saving may be achieved. To achieve thesame level of PX recovery in the downstream PX crystallizer, thecrystallizer may have to be operated at a lower temperature due thepresence of C₉+ in the C₈+ feedstock, which may need more energy tooperate the crystallizer for the same PX recovery. For a C₈+ feedstockhaving a PX concentration of greater than 70 wt % (based on totalxylenes weight in the C₈+ feedstock), a C₉+ concentration (based on thetotal C₈+ feedstock weight) of less than 10 wt %, and a naphthaleneconcentration (based on PX free C₈+ in the C₈+ feedstock) of less than10 mol %, the energy saving of eliminating the C₉+ separation towersurprisingly outweights the extra energy needed for the crystallizationunit to have equivalent PX recovery. Another advantage for theelimination of the C₉+ separation step is lower operating cost foroperating one less separation unit, which is environmental, energy, andcost beneficial.

The following examples reflect embodiments of the disclosure and are byno means intended to be limiting of the scope of the disclosure.

EXAMPLES

A general feature for p-xylene (PX) crystallization processes is thatthe product recovery is a function of the feed PX concentration and thetemperature of the coldest stage. The following equation for calculatingrecovery follows from an overall mass balance around the crystallizer:

$\begin{matrix}{R = {\frac{X_{F} - X_{E}}{1 - X_{E}} \cdot \frac{1}{X_{F}}}} & \left\lbrack {{Eq}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

where

R=fractional recovery of PX;

X_(E)=equilibrium PX content of filtrate at the coldest stage; and

X_(F)=PX content of fresh feed.

Another principle of crystallizer design is that the temperature shouldbe maintained at least 3-6° C. (5-10° F.) above the eutectic temperatureto avoid contamination of the product. Therefore, the eutectictemperature sets the coldest stage temperature, which determines X_(E)and thereby, the recovery of PX.

The following examples simulated the calculation of the eutectictemperature and potential PX recovery from the modified crystallizationprocess based on expected composition from TDP and STDP processes.

Typical C₈+ fraction composition from TDP and STDP processes are listedin Table 2 and these are the basis for the simulations in examples 1-3:

TABLE 2 Typical C₈+ composition for TDP and STDP processes Freeze PointExample 1 Example 2 Example 3 (° C.)^(1,2) (TDP) (STDP) (STDP) TypicalC₈+ composition, wt % Ethylbenzene −138.95 2.36 2.99 2.87 p-Xylene 55.8623.02 79.24 85.06 m-Xylene −54.12 47.78 10.99 6.11 o-Xylene −13.30 19.831.63 0.93 Ethyl toluene −80.16 2.71 3.30 2.12 Trimethylbenzenes −13.622.28 0.20 0.12 Indane −60.54 0.13 0.06 0.12 Propylbenzenes −140.81 0.040.00 0.06 Naphthalene 176.52 0.59 0.71 1.50 Durene³ 174.63 0.00 0.000.00 Other C₁₀ benzenes 20.75 0.03 0.00 0.12 Methylnaphthalenes 94.241.17 0.53 0.68 Other C₁₁+ 156.56 0.06 0.35 0.31 Aromatics⁴ Concentrationin PX-free filtrate, mol % Naphthalene⁵ 176.52 0.64 2.94 8.81 ¹From“Technical Data Book - Petroleum Refining”, API, 5^(th) Edition, May1992. ²The highest freeze point is listed for compound classes havingmultiple isomers. ³1,2,4,5-Tetramethyl benzene. ⁴Freeze point is listedfor biphenyl. ⁵Naphthalene mol percentage on PX free basis is calculatedbased on total mole of naphthalene divided by the total mole of C₈+ inthe feed without PX, i.e., Mol % Naphthalene in PX-free C₈+ fraction =mol % of naphthalene in C₈+/(1 - mol fraction PX in C₈+).

For purpose of simplicity, we assume that the C₉+ removal step removesall C₉+ in the C₈+ fraction. The typical C₈+ fraction composition fromTDP and STDP processes after C₉+ removal are renormalized to 100% andare listed in Table 3 and these are the basis for the simulations inexamples 1-3:

TABLE 3 Typical C₈+ composition for TDP and STDP processes after C₉+removal Typical C₈+ composition, wt % Freeze Point Example 1 Example 2Example 3 (° C.) (TDP) (STDP) (STDP) Ethylbenzene −138.95 2.54 3.15 3.02p-Xylene 55.86 24.76 83.54 89.57 m-Xylene −54.12 51.38 11.59 6.43o-Xylene −13.30 21.32 1.72 0.98 Ethyl toluene −80.16 0 0 0Trimethylbenzenes −13.62 0 0 0 Indane −60.54 0 0 0 Propylbenzenes−140.81 0 0 0 Naphthalene 176.52 0 0 0 Durene 174.63 0 0 0 Other C₁₀benzenes 20.75 0 0 0 Methylnaphthalenes 94.24 0 0 0 Other C₁₁+ 156.56 00 0 Aromatics

For the C₈ fractions, the eutectic point of interest is the temperaturewhere m-xylene (MX) co-crystallizes with PX. For the C₈+ fraction, themixture contains small concentrations of other components which havefreezing points above PX. These mixtures generally exhibit near-idealsolution behavior in which the freezing point depression of eachcomponent is dependent on its pure component properties and molefraction. Therefore, even though a component has a higher freezing pointthan PX, it will not necessarily crystallize before PX, depending on itsmole fraction in the mixture. In the examples, the limiting eutecticpoint for each mixture was determined using a modified version of thevan't Hoff equation:ln(X _(i))=−A(T*−T)[1+B(T*−T)]  [Eqn 2]

where

X_(i)=liquid phase mol fraction of component i at equilibrium

T*=freezing point of the pure component, ° K

T=mixture temperature, ° K

${A = \frac{\Delta\; H^{*}}{{R\left( T^{*} \right)}^{2}}},$a cryoscopic constant, ° K⁻¹

${B = {\frac{1}{T^{*}} - \frac{\Delta\; C_{P}}{2\;\Delta\; H^{*}}}},$a cryoscopic constant, ° K⁻¹

ΔH*=molar heat of fusion at T*, kcal/mol

ΔC_(P)=difference in molar heat capacity of liquid and solid, kcal/mol-°K

R=gas constant, 1.9872 kcal/mol-° K

Equation 2 is applicable for ideal solutions, such as the C₈+hydrocarbon fractions considered in these examples. It differs from thesimple van't Hoff equation by including a correction for the variationin enthalpy of fusion with temperature. It is therefore more accuratewhen there is a significant depression of the freezing point. Values ofthe cryoscopic constants A and B for the components of interest aregiven in Table 4:

TABLE 4 Cryoscopic constants A and B for PX, MX, OX, EB, and naphthaleneComponent A, ° K⁻¹ B,° K⁻¹ p-xylene 0.02509 0.0028 o-xylene 0.026600.0030 m-xylene 0.02742 0.0027 ethylbenzene 0.03479 0.0029 naphthalene0.01830 0.0027

Then the concentration of PX at least 5° C. above the eutectic was usedto determine the potential recovery by equation 1.

Example 1

With the C₉+ removal tower (conveniently also called C₈ tower), thesimulation shown that the MX eutectic point was limited at a temperatureof −64.3° C. Allowing for the 5° C. margin above the eutectic, the coldstage operating temperature is −59.3° C. At this temperature, estimatedrecovery for 24.8 wt % PX feed was 61.7%.

Without the C₉+ removal tower, the simulation shown that the MX eutecticpoint was limited at a temperature of −67.2° C. For the same recovery asthe case with a C₉+ removal tower, the simulation shown that therequired cold stage operating temperature was −61.3° C. Refrigerationdemands increase slightly in this case compared to operation with theC₉+ removal tower. However, the increased cost of refrigeration is morethan offset by the utility savings achieved by the elimination of theC₉+ removal tower.

Example 2

With the C₉+ removal tower, the simulation has shown that the MXeutectic point was limiting at a temperature of −63.4° C. Allowing forthe 5° C. margin above the eutectic, the cold stage operatingtemperature is −58.4° C. At this temperature, estimated recovery for83.5 wt % PX feed was 97.4%. Although the cold stage could potentiallybe operated at −58.4° C., it is advantageous in this case to design thecold stage for a higher temperature in order to eliminate the need forethylene refrigerant. The recovery at −18° C. cold stage temperature forthis case was simulated as 85.4%.

Without the C₉+ removal tower, the simulation has shown that thenaphthalene/PX eutectic point was limited at a temperature of −63.2° C.Allowing for the 5° C. margin above the eutectic, the cold stageoperating temperature is −58.2° C. At this temperature, estimatedrecovery for 83.5 wt % PX feed was 96.7%. In order to obtain the samerecovery as the case with the C₉+ removal tower (85.4%), the simulatedcold stage temperature was −23° C. Refrigeration demands increase inthis case compared to operation with the C₉+ removal tower. However, theincreased cost of refrigeration is more than offset by the utilitysavings achieved by the elimination of the C₉+ removal tower.

Example 3

With the C₉+ removal tower, the MX eutectic point is limiting at atemperature of −67.4° C. Allowing for the 5° C. margin above theeutectic, the cold stage operating temperature is −62.4° C. At thistemperature, estimated recovery for 89.57 wt % PX feed was 98.7%.Although the cold stage could potentially be operated at −62.4° C., itis advantageous in this case to design the cold stage for a highertemperature in order to eliminate the need for ethylene refrigerant. Therecovery at −18° C. cold stage temperature for this case was simulatedas 91.4%.

Without the C₉+ removal tower, the simulation shown that thenaphthalene/PX eutectic point was limited at a temperature of −32.1° C.Allowing for the 5° C. margin above the eutectic, the cold stageoperating temperature is −27.1° C. At this temperature, estimatedrecovery for 89.57 wt % PX feed was 92.0%. In order to obtain the samerecovery as the case with the C₉+ removal tower (91.4%), the simulationindicated that the cold stage temperature was reduced by 7° C. to −25°C. Refrigeration demands increase in this case compared to operationwith the C₉+ removal tower. However, the increased cost of refrigerationis more than offset by the utility savings achieved by the eliminationof the C₉+ removal tower.

TABLE 5 Summary of eutectic points, operating temperatures and PXrecovery of examples1-3 Example 1 Example 2 Example 3 Remove No C₉+Remove No C₉+ Remove No C₉+ C₉+ removal C₉+ removal C₉+ removal Eutectic−64.3 −67.2 −63.4 −63.2 −67.4 −32.1 point (° C.) Operating −59.3 −61.3−58.4 −58.2 −62.4 −27.1 Temperature (° C.) PX recovery 61.7% 61.7% 97.4%96.7% 98.7% 92.0% No ethylene N/A N/A −18 −23 −18 −25 refrigerant modeoperating Temperature (° C.) PX recovery N/A N/A 85.4% 85.4% 91.4% 91.4%for no ethylene refrigerant mode operating Temperature (° C.)

All patents and patent applications, test procedures (such as ASTMmethods), and other documents cited herein are fully incorporated byreference to the extent such disclosure is not inconsistent with thisinvention and for all jurisdictions in which such incorporation ispermitted.

When numerical lower limits and numerical upper limits are listedherein, ranges from any lower limit to any upper limit are contemplated.

The meanings of terms used herein shall take their ordinary meaning inthe art; reference shall be taken, in particular, to Handbook ofPetroleum Refining Processes, Third Edition, Robert A. Meyers, Editor,McGraw-Hill (2004). In addition, all patents and patent applications,test procedures (such as ASTM methods), and other documents cited hereinare fully incorporated by reference to the extent such disclosure is notinconsistent with this disclosure and for all jurisdictions in whichsuch incorporation is permitted. Also, when numerical lower limits andnumerical upper limits are listed herein, ranges from any lower limit toany upper limit are contemplated. Note further that Trade Names usedherein are indicated by a ™ symbol or ® symbol, indicating that thenames may be protected by certain trademark rights, e.g., they may beregistered trademarks in various jurisdictions.

While the illustrative embodiments of the disclosure have been describedwith particularity, it will be understood that various othermodifications will be apparent to and can be readily made by thoseskilled in the art without departing from the spirit and scope of thedisclosure. Accordingly, it is not intended that the scope of the claimsappended hereto be limited to the examples and descriptions set forthherein but rather that the claims be construed as encompassing all thefeatures of patentable novelty which reside in the present disclosure,including all features which would be treated as equivalents thereof bythose skilled in the art to which the disclosure pertains.

In some embodiments, this disclosure relates to:

Paragraph 1. A process of producing PX comprising providing a C₈+feedstock, said C₈+ feedstock has C₈ hydrocarbons and C₉+ hydrocarbons,to a crystallization unit under crystallization conditions to produce aPX enriched stream having a PX concentration of at least 99.5 wt % basedon the weight of said PX enriched stream,

-   -   wherein said C₈+ feedstock has a PX concentration of at least 70        wt % based on total weight of xylenes in said C₈+ feedstock,        which said C₈+ feedstock having a C₉+ hydrocarbons concentration        in a range from 1 wppm to 10 wt % based on the total weight of        said C₈+ feedstock.        Paragraph 2. A process of producing PX comprising:    -   (a) providing a toluene feedstock having toluene to a reaction        zone;    -   (b) contacting said toluene with a catalyst under toluene        disproportionation conditions to form an effluent having C₇−        hydrocarbons, C₈ hydrocarbons and C₉+ hydrocarbons, wherein said        C₈ hydrocarbons comprise PX, MX, and OX, wherein said effluent        has a PX concentration of at least 70 wt % based on total weight        of xylenes in said effluent;    -   (c) separating at least a portion of C₇− hydrocarbons from said        effluent to from a C₈+ feedstock, wherein said C₈+ feedstock has        a C₉+ hydrocarbons concentration from 1 wppm to 10 wt % based on        the total weight of the C₈+ feedstock; and    -   (d) supply at least a portion of said C₈+ feedstock to a        crystallization unit under crystallization conditions to produce        a PX enriched stream having a PX concentration of at least 99.5        wt % based on the weight of said PX enriched stream.        Paragraph 3. A process of producing PX consisting essentially        of:    -   (e) a toluene purifying step to produce a toluene feedstock        comprising at least 90 wt. % toluene and non-aromatic        hydrocarbons ranging from 1 to 10 wt % based on the weight of        said toluene feedstock; wherein said toluene purifying step has        feed(s) comprises an aromatic product stream from a catalytic        reformer, an aromatic product stream from a catalytic cracker,        and/or an aromatic product stream from a steam cracker, wherein        said aromatic product stream from a catalytic reformer, said        aromatic product stream from a catalytic cracker, or said        aromatic product stream from a steam cracker comprises at least        1 wppm to about 15 wt % non-aromatic hydrocarbons;    -   (f) contacting said toluene feedstock with a catalyst under        toluene disproportionation conditions to product a toluene        disproportionation product having light gases, Bz, PX, MX, OX,        C₉+ and unreacted toluene, wherein said toluene        disproportionating step has a toluene conversion ranging from        about 15 to 35 wt % based on the toluene in said toluene        feedstock, and wherein said toluene disproportionation product        has a PX concentration of at least 70 wt % based on total        xylenes in said toluene disproportionation product;    -   (g) separating at least a portion of said light gases, at least        a portion of said Bz, and at least a portion of said unreacted        toluene from said toluene disproportionation product to produce        a C₈+ feedstock; and    -   (h) providing said C₈+ feedstock to a crystallization unit under        crystallization conditions to produce a PX enriched stream        having a PX concentration of at least 99.7 wt % based on the        weight of said PX enriched stream, wherein said C₈+ feedstock        has a PX concentration of at least 70 wt % based on total weight        of xylenes in said C₈+ feedstock, which said C₈+ feedstock        having a C₉+ hydrocarbons concentration in a range from 5000        wppm to 10 wt % based on the total weight of said C₈+ feedstock.        Paragraph 4. The process of paragraph 2 or 3, wherein step (b)        comprises a hydrogen feed, wherein said toluene        disproportionation conditions are toluene disproportion        conditions, said toluene disproportion conditions comprise a        temperature in a range from 100 to 700° C., a pressure in a        range from 100 kPa-a to 10000 kPa-a; a WHSV in a range from        0.001 to 1000 hr⁻¹ based on the weight of said toluene in said        toluene feedstock; a molar ratio of hydrogen over toluene in a        range from 0.1 to 20.        Paragraph 5. The process of any one of paragraphs 2-4, wherein        said C₈+ feedstock is supplied to a crystallization unit without        separating C₉+ hydrocarbons from said C₈+ feedstock.        Paragraph 6. The process of any preceding paragraph, wherein        said PX concentration of at least 85 wt % based on total weight        of xylenes in said C₈+ feedstock.        Paragraph 7. The process of any preceding paragraph, wherein        said PX concentration said PX enriched stream is at least 99.7        wt % based on the weight of said PX enriched stream.        Paragraph 8. The process of any preceding paragraph, wherein        said C₉+ hydrocarbons concentration is in a range from 5000 wppm        to 2 wt % based on the total weight of said C₈+ feedstock.        Paragraph 9. The process of any preceding paragraph, wherein        said C₈+ feedstock further comprises naphthalene and said C₈+        feedstock has a naphthalene molar concentration of less than 10        mol % based on the total mole of the C₈+ hydrocarbons without        PX.        Paragraph 10. The process of any preceding paragraph, wherein        said crystallization unit is operated at a temperature of at        least −30° C.        Paragraph 11. The process of any preceding paragraph, wherein        said crystallization unit has a PX recovery of at least 85%.        Paragraph 12. The process of producing PX comprising providing a        C₈+ feedstock, said C₈+ feedstock has C₈ hydrocarbons and C₉+        hydrocarbons, to a crystallization unit under crystallization        conditions to produce a PX enriched stream having a PX        concentration of at least 99.5 wt % based on the weight of said        PX enriched stream, wherein said C₈+ feedstock is made by a STDP        process consisting essentially of:    -   (i) a toluene purifying step to produce a toluene feedstock        comprising at least 90 wt. % toluene and non-aromatic        hydrocarbons ranging from 1 to 10 wt % based on the weight of        said toluene feedstock; wherein said toluene purifying step has        feed(s) comprises an aromatic product stream from a catalytic        reformer, an aromatic product stream from a catalytic cracker,        and/or an aromatic product stream from a steam cracker, wherein        said aromatic product stream from a catalytic reformer, said        aromatic product stream from a catalytic cracker, or said        aromatic product stream from a steam cracker comprises at least        1 wppm to about 15 wt % non-aromatic hydrocarbons;    -   (j) contacting said toluene feedstock with a catalyst under        toluene disproportionation conditions to product a toluene        disproportionation product having light gases, Bz, PX, MX, OX,        C₉+ and unreacted toluene, wherein said toluene        disproportionating step has a toluene conversion ranging from        about 15 to 35 wt % based on the toluene in said toluene        feedstock, and wherein said toluene disproportionation product        has a PX concentration of at least 70 wt % based on total        xylenes in said toluene disproportionation product; and    -   (k) separating at least a portion of said light gases, at least        a portion of said Bz, and at least a portion of said unreacted        toluene from said toluene disproportionation product to produce        said C₈+ feedstock of any preceding paragraph.        Paragraph 13. An apparatus for producing a PX rich stream, which        comprises:    -   (l) a reactor having an inlet and an outlet;    -   (m) a separation unit having an inlet and a first outlet and a        second outlet, the inlet of the separation unit being fluidicly        connected to the outlet of the reactor; and    -   (n) a crystallization unit having an inlet, a first outlet, and        a second outlet, the inlet of the crystallization unit being        fluidicly connected to the second outlet of the separation unit.        Paragraph 14. The apparatus of paragraph 13, wherein said        reactor is adapted for toluene disproportionation reaction, said        inlet of the reactor is adapted to supplying a toluene feedstock        comprising toluene to said reactor to form an effluent having        C₇− hydrocarbons, C₈ hydrocarbons, and C₉+ hydrocarbons; said        outlet of the reactor is adapted to withdraw said effluent;        wherein said separation unit is adapted to separate at least a        portion of C₇− hydrocarbons from said effluent to from a C₈+        feedstock, said C₈+ feedstock has a C₉+ hydrocarbons        concentration from 1 wppm to 2 wt % based on the total weight of        the C₈+ feedstock, said second outlet of said separation unit is        adapted to withdraw said C₈+ feedstock; and wherein said        crystallization unit is adapted to produce a PX enriched stream        having a PX concentration of at least 99.5 wt % based on the        weight of said PX enriched stream.

1. In a process for producing para-xylene comprising toluene methylationto produce a stream comprising C7− hydrocarbons, xylenes, and C9+hydrocarbons, and then recovery of para-xylene by crystallization,including a step of removal of said C7− hydrocarbons from said streamprior to said crystallization, the improvement comprising the absence astep of removal of C9+ hydrocarbons between said toluene methylation andsaid crystallization.